The present invention relates to a hydraulic brake system for brake slip control and/or traction slip control having a pedal-operated master brake cylinder and wheel brake cylinders, a pressure fluid supply reservoir, a pressure fluid conduit between the working chambers of the master brake cylinder and the pressure fluid supply reservoir and at least one electromagnetically actuated value.
A hydraulic brake system is usually composed of a master brake cylinder and wheel brake cylinders connected thereto. The cylinders and the brake lines are filled with brake fluid. When the master brake cylinder is not actuated, it is arranged to connect the brake circuits to a supply reservoir for the following reasons.
On the one hand, provisions must be made so that the brake circuits are always filled completely with pressure fluid, since otherwise air bubbles will be caused in the brake circuits on operation of the brake system, which air bubbles due to their compressibility prevent a direct transmission of the pedal force onto the wheel brakes. On the other hand, heat expansion of the brake fluid must be taken into account. If the brake systems were closed-loop systems, the expansion of the brake fluid would result in force being applied on the wheel brakes, whereby the vehicle is decelerated in an undesirable way.
Currently, two methods have been devised in order to realize the connection of the brake circuits to the supply reservoir when the brake is not operated. The most simple solution resides in filling the brake system through a so-called breather bore (i.e., a transverse bore in the master brake cylinder housing) which is in communication with the supply reservoir and terminates directly in front of the working pistons of the master brake cylinder into the working chamber of the master brake cylinder. Upon operation of the brake, the working pistons displace and, in doing so, override their respectively assigned breather bore, resulting in a closed brake-circuit system on brake actuation. When the driver removes his foot from the pedal, the working pistons are reset by resetting springs into their initial position so that the connection between the working chamber and the supply reservoir is re-established via the breather bore. As long as a similar brake system is not used for controlling the brake slip, refilling of the brake circuits by way of breather bores entails no problems at all.
In anti-lock hydraulic brake systems, pressure fluid, out of an auxiliary-energy source, is introduced into the brake circuits for regulating the wheel braking pressure. As a consequence, a residual pressure is prevailing in the brake circuits and thus in the master brake cylinder when the working pistons of the master brake cylinder release the breather bores. The sealing cups of the working pistons are pressed into the breather bores by the pressure in the brake circuits and thereby destroyed. Therefore, it has been proposed to replace the breather bores by so-called central valves in anti-lock hydraulic brake systems. These valves are arranged in the working pistons of the master cylinder and normally consist of a valve ball which, in the initial position of the working pistons, is kept at a distance from the valve seat by way of a tappet abutting on a pin formed fast with the housing. When the brake is applied, and displacement of the working pistons occurs, the valve balls can move to sit on their valve seat, the brake circuits being hydraulically isolated as a result. Though this measure seems to be quite simple, considerable problems are encountered in practice. In addition, the central valves are more expensive than the construction of the breather bores, and assembly is difficult.
Moreover, both methods result in a considerably longer master brake cylinder size, since the working pistons must have at least once more the length of the working chambers.
Another disadvantage resides in that both systems have lost travel due to their construction. Before it is possible to build up pressure in the master brake cylinder, the breather bores must first be overridden, or the central valves must be closed, respectively. This lost travel implies during a braking operation that no braking effect is accomplished in the first phase of the pedal depression.
Furthermore, the working pistons must be adjusted precisely when the master brake cylinder is fitted in order to minimize the lost travel. The situation becomes particularly complicated when the master brake cylinder is operated by a booster. The booster piston, too, must be precisely adjusted so that the initial positions of the working piston and the booster piston are accurately conformed to each other.